Two-channel, dual-mode, fiber optic rotary joint

ABSTRACT

A two-channel fiber optical rotary joint has been invented in which optical signals can be transmitted simultaneously along two optical passes through a single mechanical rotational interface for both single mode fiber and multi mode fiber. The first channel of light path consists of a pair of micro-collimators, or a pair of fibers, co-axially fixed in 2 holders respectively. The light signal from one of micro-collimator, or fiber can be directly coupled into another micro-collimator, or fiber. The second channel of light path is off-axis arranged, including a pair of conventional collimators, a pair of first reflecting surfaces and a pair of second reflecting surfaces. The light signal emitted from one of the said conventional collimator will be reflected by one of the said first reflecting surface, one of the said second reflecting surface, another said second reflecting surface, and another said first reflecting surface, then coupled into another said conventional collimator. An index matching fluid is filled in the housing for lubrication and pressure compensation purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is related to two channel fiber optic rotary joint in thefield of optical transmission through a mechanical rotational interface.

2. Description of Related Art

The Fiber optic Rotary Joint (FORJ) is the opto-mechanical device whichallows uninterrupted transmission of an optical signal in a fiber guidethrough a rotational interface to a stationary apparatus. FORJ can becategorized as active and passive. An active FORJ consists of a lightsource on either rotor side or stator side and a photo detector onanother side. The disadvantage of active FORJ is requirement forelectrical power. The passive FORJ is intended to transfer opticalsignals from fiber to fiber without any electronic, or electrical units.The use of FORJ can be widely found in missile guidance systems, roboticsystems, remotely operated vehicles (ROVs), oil drilling systems,sensing systems, and many other field applications where a twist-freefiber cable is essential. Combined with electrical slip rings or fluidrotary joints, FORJs add a new dimension to traditional rotary joints.As fiber optic technology advances, more and more traditional slip ringusers will benefit from FORJs in their new fiber systems. This issue canbe solved relatively easy if only a single channel is to be transmittedbecause it can be transmitted by keeping alignment between the opticalaxis and mechanical rotational axis. However, the transmission resultsin difficulties when it is desired to transmit two channels separatelyfrom each other through a single rotation interface.

A couple of prior inventions of two channel fiber optical rotary jointare described in the following patents: U.S. Pat. No. 5,588,077, U.S.Pat. No. 4,842,355, and U.S. Pat. No. 4,725,116.

In U.S. Pat. No. 5,588,077, the two optical fiber channels are arrangedin-line along the same rotational axis. Isolation of one channel fromthe other is achieved through a novel application of gradient index rodlenses of suitable pitch. A pair of lenses is arranged adjacent eachother on each side of the rotational interface and a second pair ofaxially aligned lenses is arranged outboard of the first pair. Anoptical signal from one of the outboard lenses can be directed to one ofthe other lenses depending on the pitch selection. The drawback of thisdesign is that the losses due to crosstalk and overlap of the signalpaths would be pretty significant.

Gold, et al designed another two channel FORJ in U.S. Pat. No.4,842,355. A first channel signal is delivered to an optical fibertransmitted coaxially of the stationary and rotary side, transfer acrossthe rotational plane between the two components being accomplished byopposing centrally located optical lenses. A second channel transmittedthrough a second optical fiber is delivered to a lens system whichconverts the light into a cylinder of light coaxial with the firstchannel and which surrounds the optical management for the firstchannel. Second channel thus are converted into coaxial hollow cylindersof light. These cylinders of light are transmitted between facing lenssystems in the rotary and stationary sides of the apparatus. But thefacing lens systems are very difficult to be fabricated.

Spencer, et al shows in U.S. Pat. No. 4,725,116 a two-channel andmulti-channel FORJ. Within the joint reflecting reflecting surface areused to redirect off-axis optical signals onto the joint axis, withrelative rotation occurring while the signals are on-axis. A rotatingmember for each channel has a reflecting surface for reflecting theon-axis signal portion off-axis to a receptor fiber. Alignment betweenthe rotating member and the receptor fiber, as well as drive for therotating member, is provided by a pair of magnets of opposite polarity,one being secured to the rotating member and the other being secured tothe rotor. But it could be very difficult for the magnetic interactionto accurately ensure the synchronous rotation of the rotor and therotating member. The size of the magnetic element and the adjustment ofthe reflecting surface also increase the size of the inventedembodiment.

SUMMARY OF THE INVENTION

The first object of the present invention is to utilize the conventionalcollimators, micro-collimators, and reflecting surfaces to realize atwo-pass fiber optical rotary joints which can simultaneously transmitoptical signals through a single mechanical rotational interface with avery low-profile and compact structure for both single mode fiber andmulti mode fiber.

Another object of the present invention to minimize the need formaintaining precise axial alignment between the rotating andnon-rotating elements of a two channel fiber optic rotary joint so thatit could be used in any harsh environments such as temperature change,vibration and shock.

A further objective of the preset invention is to reduce the insertionloss and increase return loss and to allow the rotary joint to work atany ambient pressure by filling index-matching fluid.

An even further objective is to incorporate a pre-loaded precisionceramic ball bearing to achieve reduced optical losses by improvingconcentricity and long-lasting precision between the rotational andstationary elements of optic rotary joint.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section view of the first basic embodiment of theinvention.

FIG. 2 is a cross section view of second basic embodiment of theinvention.

FIG. 3 is an enlarged view of micro-rotational interface in FIG. 2.

FIG. 4 illustrates a full embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a basic design of a two-channel fiber opticalrotational interface. The holder 13 and 14 can be rotatable relative toeach other. The axis of the rotation is the geometrical axis of theholder 13 and 14. The first channel of light pass consists of amicro-collimator 11 which is fixed on the axis of holder 13, and anothermicro-collimator 12 which is fixed on the axis of holder 14. Thediameter of the micro-collimator is about 0.125 mm. Just like a singlechannel FORJ, the two micro-collimators are closely arranged opposite attheir end section and the parallel ray of light emitted from onemicro-collimator is easily transmitted to another micro-collimatorthrough the rotational interface. The second channel of light passincludes conventional collimator 15 and 16, which are fitted inside ofcollimator housing 17 and 18 respectively, and reflecting surface 19 and20. The collimator housing 17 and 18 are off -set on the holder 13 and14 respectively, with their own axis parallel to the rotational axis.The angled surface 21 on holder 13 and the angled surface 22 on holder14 are coated optical reflecting surface. The reflecting surface 19 and20 are supposed to be adhered to the angled surface of collimatorhousing 17 and 18 and parallel to the optical surface 21 and 22respectively. When the light beam emitted from one of the collimator 15,it will be reflected by the reflecting surface 19, 21, 22 and 20, thenget into another collimator. 16. vise versa. The second light pass canbe remained when the holders 13 and 14 rotate relatively. Thus atwo-channel passive FORJ is obtained. Because the diameter of theconventional collimator can be larger than 1.8 mm, the collimated beamof the micro collimator can be much smaller than that of theconventional collimator so that the lower loss and the lower cross-talkbetween the channels could be assured.

FIG. 2 shows another embodiment of the invention. It's very similar toFIG. 1. Instead of utilizing micro-collimators, a pair of optical fibersare used. The holder 133 and 144 have a through central hole at thediameter of 0.126 mm, slightly larger than the diameter of fiber 111 and122 which is 0.125 mm. Fiber 111 should be longer than fiber 122 so thatit could get into the central hole of holder 144. The end surfaces offiber 111 and 122 are oppositely positioned inside the central hole ofholder 144 on the rotational axis but separated by a clearance about 0.5um. An enlarged view inside of central hole of holder 144 is shown inFIG. 3. When the holder 133 and 144 rotates relatively each other, thefiber 111 can be rotating inside the central hole of holder 144 relativeto fiber 122. Thus forms a micro rotational interface, or “microbearing”. The micro bearing is able to compensate the mechanicalalignment error of the two fibers so that the alignment task would bejust focused on second channel. The second off-axis channel is exactlythe same as in FIG. 1. The insertion loss for the second channel can becontrolled smaller than 6 dB, the cross talk can be 50 dB, because thefiber diameter is much smaller than the light core size of secondchannel.

The mechanical details of a full embodiment of the present invention isshown in FIG. 4. The two collimator assemblies in FIG. 1, or FIG. 2 noware secured in the central holes of rotor 31 and stator 32 respectivelyby adhesive bonding. A couple of ceramic ball bearings 33 and 34 areused to allow the rotor and stator to rotate relatively each other withhigher rigidity, extended bearing life and more precision. A shaft seal36 between stator 32 and rotor 31 is mounted on the shaft 40 of rotor 31and the boring hole of seal holder 35, which is threaded with stator 32.The ceramic ball bearings, 33 and 34, are separated by an annular spacer37 and preloaded by a seal cover 35 and wave spring 43. The preloadforce should be accurately calculated to ensure the concentricity andlong-lasting static and dynamic precision between the rotor 31 andstator 32.

An index matching fluid could be used to fill in the space of stator.The shaft seal 36 and o-ring 41 are utilized to seal the assembly. Onefunction of the index matching fluid is for the lubrication betweenceramic ball bearings and the “micro bearing”. Another function of indexmatching fluid is for pressure compensating purposes. The whole spaceinside the stator 32 could be used as the pressure compensation chamber.The shaft 40 on rotor 31 is designed long enough to allow the shaft seal36 to slide axially like a pressure compensation piston when the ambientpressure is not balanced with the pressure inside the stator 32.

Although the present invention has been described in several particularembodiments of an FORJ, it is expected that additional embodiments andmodification will be apparent without departing from the spirit of theinvention.

1. A fiber optic rotary joint for optic signal transmissions comprising:A pair of relatively rotatable members: a rotor and a stator; A rotor ismounted in said stator to rotate relatively thereto through a pair ofceramic ball bearings; A first fiber optical collimator assembly beingsecured in the central hole of one of said rotatable member; A secondfiber optical collimator assembly being secured in the central hole ofanother said rotatable member; A shaft seal, a seal holder and o-ringmeans to seal the collimator assembly of the said stator and said rotorto form a sealed space with the said first fiber optical collimatorassembly and second fiber optical collimator assembly.
 2. For fiberoptical rotary joint of claim 1 wherein one each of said first fiberoptical collimator assembly and said second fiber optical collimatorassembly including a holder with an angled end surface; amicro-collimator secured in the central hole of said holder; a secondconventional collimator secured in the off-axis hole of the said holderthrough a hollow collimator housing with its own axis parallel to therotational axis; a first reflecting surface located on the front end ofsaid hollow collimator housing at a specific angle with the axis of saidconventional collimator and parallel to the said angled end surface ofsaid holder; a second reflecting surface being formed on the said angledend surface of said holder.
 3. For fiber optical rotary joint of claim 1wherein one each of said first fiber optical collimator assembly andsaid second fiber optical collimator assembly including a holder with anangled end surface; a conventional collimator secured in the off-axishole of the said holder through a hollow collimator housing with its ownaxis parallel to the rotational axis; a first reflecting surface locatedon the front end of said hollow collimator housing at a specific anglewith the axis of said conventional collimator and parallel to the saidangled end surface of said holder; a second reflecting surface beingformed on the said angled end surface of said holder; a first fibersecured in the central hole of said first holder and protruded out ofsaid central hole of said first holder; a second fiber secured in thecentral hole of said second holder and recessed inside said central holeof said second holder.
 4. For fiber optical rotary joint of claim 1 and2 wherein a first channel of light path including said micro-collimatorsco-axially fixed in said holders respectively; light signal from one ofsaid micro-collimator directly coupled into another micro-collimator; asecond channel of light path including said conventional collimators andsaid first reflecting surface and said second reflecting surface; lightsignal emitted from one of the said conventional collimator will bereflected by one of the said first reflecting surface, one of the saidsecond reflecting surface, another said second reflecting surface, andanother said first reflecting surface, then coupled into another saidconventional collimator.
 5. For fiber optical rotary joint of claim 1and 3 wherein a first channel of light path including said first fiberand said second fiber; said first fiber should be long enough toprotrude into the central hole of said second holder; the end surfacesof said first fiber and said second fiber being oppositely positionedinside the said central hole of said second holder on the rotationalaxis but separated by a very small clearance; the diameter of centralhole of said holders should be slightly larger than the diameter of saidfibers; light signal from one of said fibers directly coupled intoanother said fiber; a second channel of light path including saidconventional collimators and said first reflecting surface and saidsecond reflecting surface; light signal emitted from one of the saidconventional collimator will be reflected by one of the said firstreflecting surface, one of the said second reflecting surface, anothersaid second reflecting surface, and another said first reflectingsurface, then coupled into another said conventional collimator.
 6. Forfiber optical rotary joint of claim 2 and 4 wherein the diameter of saidmicro-collimator should be much smaller than the diameter of saidconventional collimator.
 7. For fiber optical rotary joint of claim 3and 5 wherein the diameter of said fibers should be much smaller thanthe diameter of said conventional collimator.